1. Field of the Invention
The invention relates to methods and apparatus for controlling a xe2x80x9clean burnxe2x80x9d internal combustion engine of a motor vehicle to achieve enhanced vehicle fuel economy with continued emissions compliance.
2. Background Art
The exhaust gas generated by a typical internal combustion engine, as may be found in motor vehicles, includes a variety of constituent gases, including hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NOx) and oxygen (O2). The respective rates at which an engine generates these constituent gases are typically dependent upon a variety of factors, including such operating parameters as air-fuel ratio (8), engine speed and load, engine temperature, ambient humidity, ignition timing (xe2x80x9csparkxe2x80x9d), and percentage exhaust gas recirculation (xe2x80x9cEGRxe2x80x9d). The prior art often maps values for instantaneous engine-generated or xe2x80x9cfeedgasxe2x80x9d constituents, such as HC, CO and NOx, based, for example, on detected values for instantaneous engine speed and engine load (the latter often being inferred, for example, from intake manifold pressure).
To limit the amount of feedgas constituents that are exhausted through the vehicle""s tailpipe to the atmosphere as xe2x80x9cemissions,xe2x80x9d motor vehicles typically include an exhaust purification system having an upstream and a downstream three-way catalyst. The downstream three-way catalyst is often referred to as a NOx xe2x80x9ctrapxe2x80x9d. Both the upstream and downstream catalyst store NOx when the exhaust gases are xe2x80x9cleanxe2x80x9d of stoichiometry and release previously stored NOx for reduction to harmless gases when the exhaust gases are xe2x80x9crichxe2x80x9d of stoichiometry.
Because the trap""s actual capacity to store NOx is finite, the prior art teaches vehicle emissions compliance by discontinuing lean engine operation when the trap is deemed to have been xe2x80x9cfilledxe2x80x9d to a predetermined level, thereby avoiding the condition wherein feedgas NOx would pass through a xe2x80x9cfilledxe2x80x9d trap to effect an increase in tailpipe NOx emissions. Thus, in U.S. Pat. Nos. 5,473,887 and 5,437,153, a controller accumulates estimates of feedgas NOx, all of which is presumptively stored in the trap during a given lean excursion, and triggers a purge event when the accumulated feedgas NOx exceeds a predetermined threshold representing the trap""s nominal NOx-storage capacity. And because a trap""s instantaneous efficiency, i.e., the incremental rate at which the trap stores NOx present in the exhaust gas flowing through the trap, may begin to fall as the trap fills with NOx, U.S. Pat. No. 5,437,153 further teaches use of a nominal NOx-storage capacity which is significantly less than the actual NOx capacity of the trap, to thereby theoretically provide the trap with a near-perfect instantaneous NOx-absorbing efficiency as long as stored NOx remains below the nominal capacity.
Unfortunately, this prior art approach often discontinues lean engine operation while the trap still has meaningful available NOx-storage capacity, thereby degrading the fuel economy benefit to be achieved through lean-burn operation. Even further, while increasing the frequency of the purge events may serve to maintain relatively higher trap storage efficiencies, the fuel penalty associated with the purge event""s enriched air-fuel mixture and, particularly, the fuel penalty associated with an initial release of oxygen stored previously stored in both the three-way catalyst and the trap during lean engine operation, can further degrade the fuel economy benefits associated with lean engine operation.
It is an object of the invention to provide a method and apparatus for controlling the operation of a xe2x80x9clean-burnxe2x80x9d internal combustion engine which seeks to maximize the fuel economy benefit from lean engine operation while providing overall emissions compliance.
In accordance with the invention, a method and apparatus are provided for controlling the operation of a lean-burn engine of a motor vehicle, wherein the engine generates exhaust gas including a first constituent gas, such as NOx, and wherein exhaust gas is directed through an exhaust gas purification system including an emissions control device that is operative to store an amount of the first constituent gas when the engine is operated in a first operating condition, and is further operative to release a previously-stored quantity of the first constituent gas when the engine is operating in a second operating condition. Under the invention, the method includes determining a first measure representative of a total amount of the first exhaust gas constituent exhausted to the atmosphere during a first engine operating period; determining a second measure representing a distance traveled by the vehicle during the first engine operating period; and calculating a third measure based on the first measure and the second measure. The method further preferably includes comparing the third measure to a predetermined threshold, and discontinuing lean engine operation when the third measure exceeds the predetermined threshold. In an exemplary embodiment, the predetermined threshold is periodically adjusted based upon an indication of vehicle activity determined, for example, based upon actual engine power.
In accordance with another feature of the invention, determining the first measure includes determining a first value representative of an instantaneous amount of the exhaust gas constituent being exhausted to the atmosphere, for example, based upon an output signal generated by a sensor positioned downstream of the device; and accumulating the first value during the first engine operating period. In an exemplary embodiment, the first value is determined as a function of instantaneous operating conditions and an instantaneous device storage efficiency.
In accordance with a further feature of the invention, determining the second measure includes detecting a value representative of instantaneous vehicle speed, with the detected instantaneous vehicle speed value being clipped at a predetermined, nonzero minimum value. The predetermined, nonzero minimum value preferably approximates a minimum vehicle speed characterized by a level of constituent emissions that is at least as great as the levels of constituent emissions generated by the engine when idling at a stoichiometric operating condition.
Other objects, features and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.